The present invention relates to a silica based optical waveguide provided with a core waveguide and an over cladding part on a substrate, to be used for the optical communication field, and a production method therefor.
A silica based optical waveguide comprising a core waveguide made of a silica based glass with a rectangular or square lateral cross-section with a refractive index increasing dopant such as GeO2 added, having a relatively high refractive index formed on a substrate made of a silica, silicon, or the like, and an over cladding part made of a silica based glass, having a refractive index lower than that of the core waveguide formed on the substrate so as to cover the core waveguide, is well known. Moreover, the over cladding part of the silica based optical waveguide is produced by, in general, accumulating silica based glass fine particles on the substrate provided with the core waveguide by Flame Hydrolysis Deposition (FHD) method so as to form a porous silica based glass layer, and sintering the same so as to be transparent.
Moreover, in order to prevent deformation of the core waveguide by the heat at the time of sintering the porous silica based glass layer, a dopant such as P2O5, and B2O3 is added to SiO2 in the over cladding part for lowering the softening temperature such that the softening temperature of the over cladding part is made lower than the softening temperature of the core waveguide. The refractive index of the over cladding part should be lower than the refractive index of the core waveguide so as to be substantially same as the refractive index of the substrate so that the refractive index increasing effect of P2O5 and the refractive index lowering effect of B2O3 offset with each other.
According to the silica based optical waveguide, due to the difference in the thermal expansion coefficients in the substrate and the over cladding part surrounding the core waveguide, anisotropic thermal distortion is generated in the core waveguide so that the polarization dependency is generated. According to an optical wavelength multiplexer and demultiplexer comprising the silica optical waveguide, since the multiplexing and demultiplexing characteristics are changed according to its polarization dependency, a silica based optical waveguide with least polarization dependency is called for. JP-A-6-27342 and JP-A-7-318734 disclose a method for reducing the polarization dependency of a silica based optical waveguide. (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d.)
The method disclosed in JP-A-6-27342 is for reducing the polarization dependency by having the total doping amount of P2O5 and B2O3 in the over cladding part by 1 to 3%. Moreover, the method disclosed in JP-A-7-318734 is for reducing the polarization dependency by forming a thin film glass layer having a refractive index smaller than that of a core waveguide and same as or more than that of a substrate, on the border surface between the over cladding part and the substrate or the core waveguide.
However, if the total doping amount of P2O5 and B2O3 in the over cladding part is reduced as disclosed in JP-A-6-27342, crystals may be generated on the interface between the core waveguide and the over cladding part. Since the crystal generation brings about a great adverse effect to the propagating loss as the optical waveguide, it should be avoided. Moreover, according to the method disclosed in JP-A-7-318734, the refractive index of the thin film glass layer is larger than that of the over cladding part. Therefore, if the thickness of the thin film glass layer is large, a plurality of the core waveguides cannot be disposed parallel with a narrow interval in the optical waveguide, and thus it is difficult to reduce the total size of the optical waveguide. Moreover, if the P2O5 concentration is lowered so as to lower the refractive index of the thin film glass layer, crystals may be generated on the interface with respect to the core waveguide.
Accordingly, in order to solve the problems of the prior art, the invention is to provide an optical waveguide with little polarization dependency, capable of achieving a high density arrangement of a core waveguide by eliminating crystals generated on the interface between the core waveguide and the over cladding part and preventing formation of a thin film layer having a refractive index larger than that of the other part in the over cladding part, on the interface between the core waveguide and the over cladding part, and a production method therefor.
A silica based optical waveguide according to the invention comprises a substrate, a core waveguide formed thereon, and an over cladding part comprising a silica based glass with a refractive index lowering dopant and a refractive index increasing dopant added, formed on the substrate so as to cover the core waveguide, wherein a segregation layer with a higher concentration of the refractive index increasing dopant is formed in a part of the over cladding part in contact with the substrate and the core waveguide such that at least a part of the refractive index increase in the segregation layer provided by the refractive index increasing dopant with respect to the part of the over cladding part other than the segregation layer is offset by decline of the refractive index by increasing the amount of the refractive index lowering dopant added in the segregation layer and/or adding another refractive index lowering dopant.
Accordingly, since the segregation layer with a higher concentration of the refractive index increasing dopant is formed in the part of the over cladding part in contact with the substrate and the core waveguide, crystal generation can be restrained. Moreover, since at least a part of the increase of the refractive index in the segregation layer is offset by decline of the refractive index by increasing the amount of the refractive index lowering dopant added and/or adding another refractive index lowering dopant, the refractive index of the part of the over cladding part in contact with the core waveguide can be substantially same as the other part of the over cladding part, and thus transmitting light beams can hardly interfere with each other even in the case of a high density arrangement of a plurality of core waveguides parallel with a narrow interval.
In the present invention, the refractive index increasing dopant is preferably P2O5, the refractive index lowering dopant is preferably B2O3, and the other refractive index lowering dopant is preferably fluorine.
Furthermore, an optical waveguide according to the invention can be produced as follows. First, after forming a core glass film on a substrate, a core waveguide with a rectangular or square lateral cross-section is formed from the core glass film by a photolithography, a reactive ion etching, or the like. By accumulating glass fine particles comprising a silica glass with a refractive index lowering dopant and P2O5 added by FHD method on the substrate so as to cover the core waveguide while keeping the substrate with the core waveguide formed at a 500xc2x0 C. to 700xc2x0 C. temperature, a porous silica based glass layer having a segregation layer with P2O5 segregated with a higher concentration in the part in contact with the substrate and the core waveguide is formed.
Next, the porous silica based glass layer is heated in a fluorine gas atmosphere so as to offset the refractive index increase in the segregation layer derived from the segregation of P2O5 by the refractive index decline by segregation of fluorine, and subsequently the porous silica based glass layer is sintered so as to provide a transparent vitrified over cladding part. Moreover, instead of offset the increase of the refractive index derived from the segregation of P2O5 by the refractive index decline by fluorine, it is also possible to offset the increase of the refractive index derived from the segregation of P2O5 by increase of the refractive index lowering dopant amount by adjusting the material supply amount of the refractive index lowering dopant in the part so as to increase the dopant amount. Moreover, it is also possible to offset the entire refractive index increase derived from the segregation of P2O5 by employing both offset by fluorine and offset by the refractive index lowering dopant amount increase.